Global demand for clean fuels, such as ultra-low-sulfur-diesel (ULSD), has risen quickly as many governments have enacted environmental regulations that require substantially lower sulfur levels for cleaner burning or simply “clean fuels”, in order to reduce sulfur dioxide (SO2) emissions from use of such fuels.
Hydroprocessing processes, such as hydrodesulfurization (HDS) and hydrodenitrogenation (HDN), which remove sulfur and nitrogen, respectively, have been used to treat hydrocarbon feeds to produce clean fuels.
Conventional three-phase hydroprocessing reactors, commonly known as trickle bed reactors, require transfer of hydrogen gas from the vapor phase through a liquid-phase hydrocarbon feed to react with the feed at the surface of a solid catalyst. Thus, three phases (gas, liquid and solid) are present. The continuous phase through the reactor is the gas phase. Trickle bed reactors can be expensive to operate. They require use of a large excess of hydrogen relative to the feed. Excess hydrogen is recycled through large compressors to avoid loss of the hydrogen value. In addition, significant coke formation causing catalyst deactivation has been an issue due to localized overheating as trickle bed operation can fail to effectively dissipate heat generated during hydroprocessing.
Ackerson et al. in U.S. Pat. No. 6,123,835, disclose a two-phase hydroprocessing system which eliminates the need to transfer hydrogen gas from the vapor phase through a liquid phase hydrocarbon to the surface of a solid catalyst. In the two-phase hydroprocessing system, a solvent, which may be a recycled portion of hydroprocessed liquid effluent, acts as diluent and is mixed with a hydrocarbon feed. Hydrogen is dissolved in the feed/diluent mixture to provide hydrogen in the liquid phase. Substantially all of the hydrogen required in the hydroprocessing reaction is available in solution.
Kokayeff et al. in U.S. Patent Application Publication No. 2009/0321310 disclose a process which combines a substantially liquid-phase (two-phase) hydroprocessing zone with a substantially three-phase hydroprocessing zone in a manner such that the hydrogen requirements for both reaction zones is provided from an external source to the three-phase zone. Kokayeff et al. defines “substantially liquid-phase” as including up to 5000 percent of saturation. The use of hydrogen recycle or a recycle gas compressor is considered unnecessary and can be eliminated. The effluent from the three-phase zone contains excess hydrogen and is directed to the liquid-phase zone, where the hydrogen present in the effluent satisfies the hydrogen requirement for the liquid phase reactions. To facilitate flow of hydrogen gas from the three-phase zone to the liquid-phase zone, Kokayeff et al. preferably operates the three-phase zone at a higher pressure than the liquid-phase zone.
While Kokayeff et al. seek to combine advantages of liquid-phase (two-phase) hydroprocessing with three-phase hydroprocessing, challenges remain due to effectiveness of the liquid-phase zone by relying on the three-phase zone for hydrogen. Conversion in the liquid-phase zone may be limited due to hydrogen solubility, so that substantial conversion may be needed in the three-phase zone, that is large reactor(s), to meet desired conversion.
It remains desirable to provide an efficient process for hydroprocessing hydrocarbon feeds, which provides a high conversion in terms of sulfur and nitrogen removal, density reduction, and cetane number increase. It is desirable to combine the economy of a liquid-phase process which may use smaller reactors with the effectiveness of a three-phase process which may provide high conversions in kinetically limited regions. It also remains desirable to have a hydroprocessing process to produce a product that meets a number of commercial transportation fuel requirements, including Euro V ULSD specifications.